1.Field of the Invention
The present invention relates generally to an exposure apparatus and method and, more particularly, to an exposure apparatus for a large-sized substrate such as a liquid crystal display panel.
2. Related Background Art
Display quality of the liquid crystal display panel has been remarkably improved in recent years. Use of liquid crystal display panels has been widespread because of its advantages of being small both in thickness and weight, in place of the CRT. There have been advancements in colorization and scale-up of a direct-view type liquid crystal panel especially by an active matrix method, and glass substrates employed for manufacturing this panel have increased in size.
Exposure methods for exposing such a large-sized substrate (photosensitive substrate) include a so-called proximity method, a step-and-repeat method and a mirror projection method. The proximity method involves batch-illumination of a mask and a photosensitive substrate that are in close proximity to each other. In the step-and-repeat method, a projection optical system is constructed as an equi-powered lens image forming system providing a large transfer area.
In the mirror projection method, the projection optical system is constructed as an equi-powered mirror image forming system, and a scan exposure is performed by projecting a mask image in a circular arc on the photosensitive substrate.
When the exposure based on the proximity method is used for the large-sized substrate, it is required that the large-sized mask and the photosensitive substrate be made as close as several tens of xcexcms. Proximity spacing largely influences configuration, line width and resolution of the image to be transferred. It is quite difficult to transfer the mask pattern with no defect onto the entire surface of the photosensitive substrate because problems with flatness of the mask and the photosensitive substrate, protrusions of a resist coated on the photosensitive substrate and dust on the surface make it difficult to achieve close proximity. Thus the proximity method is unsuited to the liquid crystal panel based on the active matrix method. For this reason, the proximity method is employed for the liquid crystal panel oriented exposure apparatus according to a TN (Twisted Nematic) method and an STN (Super-Twisted Nematic) method.
The step-and-repeat method involves the use of a reticle on the order of 6 inches, which is relatively small as compared with the photosensitive substrate, and a transfer onto the large-sized substrate is effected by the step-and-repeat process. In the step-and-repeat method, a reticle used for semiconductor exposure can be employed as an original, and, therefore, it is possible to apply techniques developed for the semiconductor exposure, such as management of writing accuracy and pattern dimensions and also management of dust. However, when an image is transferred onto a large-sized substrate, in the case of an exposure of a device having a large area that the projection optical system can not cover, a so-called split exposure is required to be conducted, wherein the device is split into small area, and the exposure is performed while joining these areas to each other on the photosensitive substrate.
In this split exposure, if a trace of positional deviation and a variation in density are produced at a boundary portion (split portion) of patterns undergoing the split exposures on the display region of the liquid crystal panel based on, especially, the active matrix method, such differences are perceived by sensitive human sight when a device is completed. It may happen that this causes a problem in terms of the display quality. Besides, if the number of split areas increases, the reticle has to be replaced many times when a single photosensitive substrate is exposed, and it follows that processing capability is decreased.
In the mirror projection method, the transfer is carried out by an equi-powered system, and, hence, there is needed a large-sized mask corresponding to a size of the photosensitive substrate. Writing of the large-sized mask becomes practicable, however, there are many problems to be obviated. Since the exposure is effected by scanning a circular-arc slit on the photosensitive substrate, it is therefore required that slit length be elongated according to the scale-ups of the photosensitive substrate and the liquid crystal panel. Consequently, the mirror-based equi-powered image forming system has to be increased in size, resulting in a scale-up of the optical system. This causes a remarkable increment in the cost of the apparatus.
It is an object of the present invention to provide an exposure apparatus and method capable of performing an exposure on a large-sized substrate without splitting a device into small areas, wherein an original involves the use of, e.g., a reticle.
To accomplish the above object, according to the present invention, not a large-sized mask but a reticle on the order of 6 inches is employed as an original, and a projection optical system involves the use of a magnifying image forming system set to a magnification determined by a size of whole patterns (pattern area) of the reticle and a size of an exposed area on a photosensitive substrate so that the whole patterns of the reticle correspond to the exposed area on the photosensitive substrate. Further, an area of reticle patterns transferred onto the photosensitive substrate by the magnifying image forming system is enlarged, and, at the same time, the entire surface of the photosensitive substrate is exposed without exposing the whole patterns of the reticle on the photosensitive substrate at one time, because of a relationship with an image circle (effective projection area) of the projection optical system. The invention employs a scan exposure method in which whole patterns of the reticle are partially sequentially exposed on the photosensitive substrate. For this reason, there arises a necessity for performing a scan in which the reticle and the photosensitive substrate are related in a predetermined relationship with respect to the magnifying projection optical system. A position detecting device for detecting a position of the reticle and a position detecting device for detecting a position of the photosensitive substrate are provided. At the same time, items of positional data of the reticle and the photosensitive substrate are obtained from these position detecting devices. A drive device is provided for moving the reticle and the photosensitive substrate while keeping the above predetermined relationship. The drive device moves the reticle and the photosensitive substrate on the basis of the positional data thereof inclusively of data about aberrations, etc. of the projection optical system.
Further, a preferable range i s as follows:
1.5xe2x89xa6xcex2xe2x89xa64.0
where xcex2 is the magnification of the projection optical system. If under a lower limit, an exposure area with respect to the photosensitive substrate becomes too small, and consequently a necessity for splitting arises. Also, if above an upper limit, no matter how high the accuracy with which patterns of the reticle original can be created may be, the magnification is too large. It is therefore difficult to keep the accuracy of the pattern to be transferred onto the photosensitive substrate due to the aberrations of the projection optical system.
According to the present invention, the reticle patterns are sequentially exposed in an area defined by an image circle of the projection optical system on an exposed surface of the photosensitive substrate. With an illumination system and the projection optical system fixed to the exposure apparatus, the drive device moves the reticle and the photosensitive substrate on the basis of the positional data of the reticle and the photosensitive substrate so that a projection area on the reticle pattern is exposed exactly on a corresponding projected area on the exposed surface of the photosensitive substrate. As described above, according to the present invention, a reticle is employed as the original, and it is therefore possible to use techniques established by semiconductor exposure with respect to the management of the writing accuracy and pattern dimensions and also the management of the dust. Consequently, a high exposure accuracy is exhibited.
Further, the projection optical system is constructed as a magnifying image forming system, and the reticle and the photosensitive substrate are scanned with respect to the projection optical system. Exposure on a large photosensitive substrate can be thereby performed with even a small original without making the image circle of the projection optical system larger than needed. Simultaneously, the scale-up of the projection optical system can be avoided.
That is, a small-sized reticle on the order of 6xe2x80x3-7xe2x80x3 can be employed as a mask for the exposure. Further, a non-split exposure can be performed on a large-sized substrate by scanning, wherein the projection optical system is formed as a magnifying system. Moreover, the effective projection area of the magnifying projection optical system may be relatively small as compared with a size of the substrate, hence, avoiding increase in size of the projection optical system.